A number of different systems and methods have been developed for delivering electrical shocks to a patient's heart in response to detected abnormal heart rhythms (arrhythmias). These methods deliver specific waveform shapes or pulse sequences to the heart in order to treat the detected arrhythmia. One early waveform is disclosed in U.S. Pat. No. 3,706,313 to Milani et. al., which provides a circuit for delivering a "trapezoidal" wave shape for defibrillating the heart by truncating the output of an exponentially decaying capacitor. Others have suggested the use of sequential pulses delivered through multiple pathways such as described in U.S. Pat. No. 4,708,145 to Tacker, Jr. In Tacker, Jr., a series of rectangular or truncated exponential pulses are delivered to the heart using at least three electrodes where a first pulse is sent through a first pair of the three electrodes and then a second pulse is sent through a second different pair of the electrodes. Still others have described the use of multiphasic waveforms such as U.S. Pat. No. 4,637,397 to Jones et. al. which describes a triphasic waveform. A triphasic waveform has three pulses of alternating positive and negative polarity. U.S. Pat. No. 3,924,641 to Weiss and U.S. Pat. No. 4,850,357 to Bach, Jr. describe the use of biphasic waveforms. These defibrillation pulses are typically in the range of from 500 to 1000 volts delivered for a time of from about 2 to 10 msec. Overall energy delivery for a defibrillation waveform may typically be from about 10 to 30 joules.
There has been much debate over the optimum waveform, i.e. which waveform is most effective from a therapeutic standpoint and also most efficient from an energy delivery standpoint. The primary goal in treating a detected fibrillation with an implantable cardioverter/defibrillator is to minimize energy delivery requirements for the defibrillation waveform by providing the most effective therapy with the lowest energy. Lower voltage shocks are less painful and disruptive to the patient and lower energy requirements allow for use of smaller batteries and capacitors and thus smaller implantable devices.
A modification of the standard waveform has been suggested by Imran in U.S. Pat. No. 4,768,512. That patent discloses a cardioverting system (defibrillation and cardioversion) in which a truncated exponential waveform is chopped at high frequencies to provide a voltage wave packet formed of a plurality of high-frequency cardioverting pulses with a preferred frequency in excess of 1 KHz. Iraran teaches that different heart tissues have different impedances at different frequencies and that tissues of high and low impedance are distributed throughout the heart. Thus, with high-frequency pulses, the energy is distributed throughout the heart resulting in lower energy requirements for effective cardioversion.
The electrical activity of the heart reflects the activity of a dynamical system. A dynamical system is a system which may be described with differential equations having at least three independent dynamical (time dependent) variables and the equations must contain a nonlinear term which couples several of the variables. This coupling is a manifestation of feedback. Dynamical systems such as the heart can exhibit both periodic and chaotic behavior depending on certain system parameters. These parameters appear as constants in the differential equations describing the system. The chaos exhibited by the heart may not be immediately obvious by looking at an ECG. One standard way of observing the chaotic behavior of the heart has been to plot the interbeat spacing at time n against the interbeat spacing at time n+1. Such a plot is known as a Poincare map or return map. However, it has been discovered that a better variable for representing the dynamical system of the heart is the amplitude of the ECG. FIG. 1A shows such a plot for a human heart as it goes from a normal heart rhythm to the transition to the chaos of fibrillation. The plot covers about 8 seconds with 4000 data points each taken at 2 milliseconds apart. For the particular ECG of this example, the delay time for the return map has been found to be 088 seconds or 44 data points. Thus, the figure shows the amplitude of the ECG at time n plotted against the amplitude at time n+44. The plot shows a pattern with a high degree of organization for normal heart rhythm until the transition to chaos. FIG. 1B shows a continuation of about 4 seconds of the plot for the ECG used in FIG. 1A as the heart is fibrillating, clearly exhibiting chaotic behavior. This technique of viewing the heart dynamical system can thus provide an improved mechanism for interpreting the behavior of the heart.
It has been shown that a chaotic system can be controlled by continuously applying proportional feedback to the system. This technique has been described with respect to laser systems in "Method of controlling chaos in laser equations", Phys. Rev. E, Minh Duong-van, Vol. 47, No. 1, pp. 714-717, Jan. 1993. This type of continuous control for treating cardiac arrhythmias has been proposed by Garfinkel et. al. in SCIENCE Vol. 257, pp. 1230-1235, 28 Aug. 1992. However, the proposed technique may be undesirable because it requires continuous therapy. If the therapy is discontinued, the heart returns to fibrillation.
It is an object of the present invention to provide an improved o defibrillation waveform which allows effective therapy with lower energy delivery requirements.
It is a further object of the invention to utilize the chaotic behavior of the heart system to provide a more efficient defibrillation therapy.